The Indonesian Archipelago is one of the most complex geological region of the world. It achieved its present shape 10 to 15 million year ago in the mid-Miocene epoch of the tertiary era, as the culmination of the long process of continental fragmentation and drift. The present day situation is far from settled as the underlying strata of the earth are still in a state of continuous flux. Occasionally these feature are brought to our attention in the form of volcanic eruption and earthquake- regular features in many parts of the Archipelago.
A changing World – Division and Collision
About 180 million years ago the world was made up of huge continent called Laurasia, comprising North America, Europe and Asia (as far east as Sulawesi), and the southern one called Gondwanaland, which included South America, Africa, India, Australia, New Guinea and Antarctica.
Arcs of islands formed at the edges of Laurasia and Gondwana, separated from the parent continents by basins created by the spreading sea floor. The convoluted chains of islands in central and eastern Indonesia today include the visible remains of this arcs. With time, these have been distorted by further collisions and modified by additional spreading of the sea floor and by the sliding of some fragments below others – a process called seductions – in the course of which lines of volcanoes have developed.
As a result, Laurasia now extent into Indonesia as the Sunda continental shelf on which lie Sumatra, Java, Bali and Borneo, while the Gondwana section is represented by the Sahul shelf on which New Guinea and adjacent islands rest. Between these relatively shallow shelves, the islands of central Indonesia are separated by much deeper seas.
The Rise and Fall of Sea Levels
The present day sea level is within a few meters of the highest level ever reached during the entire Pleistocene period – two million years ago. On several occasions, such as at the peak of numerous glacial periods, the sea was as much as 180 meters below its present level. At one stages, a great river flowed in what is now the South China Sea between Peninsular Malaysia and Borneo; its drowned valley can still be detected. When the sea level was low, the Sunda and Sahul continental shelves were exposed as dry land and they played a central role in the movement of plants and animals between the different islands.
Wildlife of Indonesia
The fauna and flora of archipelago, its diversity as well as distribution of certain species, reflect some of the geological changes that have taken place in this region. The fauna of Irian Jaya, for example, demonstrated strong Australian affinities, with a large number of marsupials represented island of the western region, In contrast are largely populated by species with distinct Asian characteristic; many species found to day on Sumatra, Java and Kalimantan are similar to those of mainland Southeast Asia, revealing the fact that land masses were all formerly joined.